During evolution viruses have developed an array of mechanisms to escape surveillance by the immune system, including production of decoy proteins that mimic the host's molecules for the purpose of avoiding nonself recognition. The murine cytomegalovirus (MCMV) genome codes for an MHC class I-like molecule, m157, which binds the inhibitory NK cell receptor Ly49I. The m157 protein is displayed on the surface of MCMV-infected cells to prevent NK cell attack. We will carry out biophysical and structural studies of the interaction of the inhibitory receptor Ly49I with the viral protein m157. The immune system counterattacks with activating NK receptors, such as Ly49H, that activate NK cells once they encounter m157 decoy molecules on the surface of target cells. This interaction protects the host against viral infection by killing the infected cell. We will conduct research to understand the biophysical and structural basis for the interaction of the activating receptor Ly49H with the MCMV protein m157. In addition, MCMV somehow modifies the MHC class I molecule H-2Dk expressed on an infected cell in a manner that induces NK cells carrying the activating Ly49P receptor to kill the target cell. To understand the nature of this mechanism, whereby "modified self" resists a pathogen infection, we will determine the crystal structures of the activating receptor Ly49P in complex with H-2Dk loaded with viral or self peptides. KLRG1, a lectin-like receptor found in both human and mouse, is an inhibitory NK cell and CD8+ T cell receptor over-expressed during viral infection. We will analyze the binding properties and determine the crystal structure of KLRG1 in complex with its cognate ligands, the adhesion molecules N-, R- and E-cadherin. The KLRG1-cadherin interaction is believed to play a role in the killing of virally infected and metastatic cancer cells. We expect that the proposed studies will define the molecular basis for the interaction of NK cell receptors with both viral (m157) and host (MHC class I, cadherins) ligands that regulate NK cell activity during the course of viral infections. This research will be done primarily in Argentina at the University of Buenos Aires in collaboration with Dr. Emilio Malchiodi, as an extension of NIH Grant R01 AI047990-07 (Mariuzza; 7/1/2004-6/30/2009). More than 3 billion people today live with cytomegalovirus (CMV, Herpesviridae) infection. Viral and host mechanisms have coevolved to enable the pair to live in a mutually tolerant relationship in most infected individuals, but the virus is pathogenic in neonatal life and in adults who are immunocompromised. Natural killer (NK) cells play a central role in innate immune responses to a variety of virally infected or malignant cells. NK cell function is regulated by a dynamic balance between activating and inhibitory receptors that interact with specific ligands. The goal of this proposal is to obtain structural insights into the interaction of murine NK cell receptors with viral or host ligands that regulate NK cell function during the course of viral infections. Such knowledge may contribute to the development of new strategies for the treatment of viral infections based on modulation of NK cell cytolytic activity. [unreadable] [unreadable] [unreadable]